Development of green electroluminescent terbium doped zinc sulfide thin film phosphors

Alternating current thin film electroluminescent (ACTFEL) phosphors based on ZnS:Tb were fabricated by R.F. magnetron sputtering. The brightness and efficiency of the devices were improved by controlling deposition parameters such as target configuration and sputtering gas composition, in addition to codoping using Ag, Cu and Ce. By using a ZnS:TbOF single pressed power target, the brightness at 40V above threshold (B₄₀) was improved to 89 cd/m2 compared to 44 cd/m2 for films deposited from two separate ZnS and TbOF targets. This increase of brightness is attributed to improved crystallinity due to reduced negative ion resputtering (NIR) effects from O− and F−. Sputter-deposition of ZnS:TbOF films using Ar + He mixture (the He gas concentration was changed from 0% to 70%) also improved the crystallinity of deposited film. However, the brightness of the device decreases from 64 cd/m2 for films deposited with pure Ar atmosphere to 45–54 cd/m2 for 60–70% He gas concentrations. This brightness decrease is attributed to an increased dielectric constant (from 12 for pure Ar to 17 at 70% He) and decreased RMS surface roughness (from 6.7 nm for pure Ar to 4.3 nm at 70% He). To investigate the effects of oxygen incorporation, ZnS:Tb,F films were deposited from a ZnS:TbF₃ target with an oxygen flow. ZnS:Tb,F films show the best brightness (82 cd/m2) at 3.6 at. % of oxygen concentration in the deposited film, with a very sharp drop off in brightness from either underdoping (56 cd/m2 at 2.2 at. % oxygen) or overdoping (42 cd/m2 at 8.1 at. % oxygen). The brightness improvement by oxygen incorporation can be attributed to an increased conduction charge. The brightness decrease by oxygen overdoping is attributed to both a decreased excitation and a decreased radiative efficiency. After annealing at 500°C for 60 minutes, Ce codoping increases the B₄₀ to 144 cd/m 2 compared to 86 cd/m2 of uncodoped ZnS:TbOF films. This brightness improvement is attributed to an increased radiative efficiency. At V₂₀, the radiative efficiency of Ce codoped films is improved by ≈130%. The non-linear exponential decay of the electroluminescent emission for Ce codoped films is attributed to an energy transfer from Ce+3 to Tb+3 luminescent centers. After annealing at 500°C for 60 minutes, Ag + Cu codoped ZnS:TbOF thin film phosphor also showed increased brightness at 20V above threshold, B20 of 67 cd/m2 compared to 55 cd/m 2 for uncodoped ZnS:TbOF films. The increased brightness for Cu + Ag codoped ZnS:TbOF film is attributed to increased surface roughness and improved excitation efficiency caused by space charge modification due to Ag+1 and Cu+1 codoping.